Surface oxidation of molten AZ91D magnesium alloy in air

Abstract

The surface of a stagnant AZ91D alloy melt has been exposed to air under two oxidation conditions: at 610°C for 30 min and at 650°C for 10 s. After oxidising, the samples were sand quenched. The macroscopic morphology of the surfaces exhibited regions of non-nodular growth (in a layer manner) and regions of nodular growth. Additionally, the 610°C 30 min condition presented a region where localised combustion took place. The microscopic morphology of the surfaces and the nature of the surface reaction products have been analysed using field emission gun SEM and X-ray diffraction techniques, respectively. These results showed that all these morphologies, i.e. sponge-like, nodular and layer, were porous and therefore non-protective with respect to the evaporation and oxidation of Mg and Zn from the surface of the AZ91D alloy melt. In all these regions, MgO was the main oxidation product along with traces of ZnO and AlN.

Keywords:

• Magnesium alloys
• Oxidation
• High temperature
• Liquid state

Acknowledgements

The financial support of the Engineering and Physical Sciences Research Council (EPSRC) is gratefully acknowledged. Thanks are due to the Experimental Techniques Centre (ETC) for the provision of the FEG-SEM and XRD research facilities at Brunel University London and their assistance in this work.